A typical optical transceiver module currently used in optical communications includes a transmitter portion and a receiver portion. The transmitter (TX) portion includes a laser driver, which is typically an integrated circuit (IC), one or more laser diodes, and an optics system. The laser driver outputs electrical signals to the laser diodes to modulate them. When the laser diodes are modulated, they output optical signals, which are then directed by the optics system of the TX portion onto the ends of respective transmit optical fibers or waveguides held within a connector that mates with the transceiver module. The TX portion typically also includes an open loop or closed loop optical output power control system for maintaining the average optical output power levels of the lasers at substantially constant levels.
The receiver (RX) portion of the optical transceiver module typically includes at least one photodetector, at least one transimpedance amplifier (TIA), and at least one linear amplifier (LA). The photodetector, which is typically a P-inversion-N (PIN) photodiode, produces an electrical current signal in response to light detected by the photodetector. The TIA receives a single-ended voltage signal from the photodetector and compares the voltage signal to a slicing threshold (TH) voltage level and produces a differential voltage signal. This differential voltage signal is then input to the LA. The LA is a high gain differential amplifier that quantizes or digitizes the differential voltage signal output from the TIA.
In a typical optical link, the optical link budget is a key factor in determining how well the optical link will perform. The maximum amount of optical loss that can be tolerated in the link is dependent on the minimum amount of optical transmit power that can be guaranteed and on the minimum amount of optical power that can be detected (receiver sensitivity). The link performance is also limited by the maximum power that the receiver can tolerate before the receiver experiences a condition known as receiver overload. This condition occurs when the maximum transmit power within eye safety limits is being transmitted over the link while the minimum amount of optical loss is occurring in the link. The overall dynamic range available in the optical receiver often limits the design of an optical system.
In the typical TIA used in an optical RX module, tradeoffs are made between gain, bandwidth, power dissipation and overload levels of the TIA. The sensitivity of an optical TX module is most significantly influenced by the noise, gain and bandwidth of the input stages of the TIA. The circuit noise is minimized by optimal sizing and biasing of the transistors of the TIA. The bandwidth of a TIA for a given process technology is affected by sizing and biasing of the transistors of the TIA. In general, an increase in the gain of the TIA, for a given amount of power dissipation, results in a decrease in the bandwidth of the TIA. Increasing the gain of the TIA reduces the input-referred noise, but reduces the bandwidth of the TIA. The reduced bandwidth is recoverable by increasing the bias current of the transistors of the TIA or by using novel circuit topologies. Increasing the gain of the TIA, however, can result in voltage swings in the TIA that cause receiver overload.
The upper limit of the voltage swing in the TIA is most heavily influenced by the following two factors, namely, voltage headroom and device breakdown voltage. The voltage headroom is set by the supply voltage of the TIA. Higher supply voltages result in more headroom, which allows for larger voltage swings before transistors are pushed into saturation (or out of saturation when referring to complementary metal oxide semiconductor (CMOS) devices). Receiver overload is observed when bipolar devices are pushed into saturation or CMOS devices are pushed into linear operation. When this happens, a large increase in jitter is observed, which is due to a decrease in the speed of the data transitions.
Device breakdown voltage refers to the breakdown voltage of the transistors that make up the TIA. For transistors made using advanced process technologies, device breakdown voltage presents a limit that cannot be overcome. Creative circuit techniques can be used to bias transistors in analog circuits so that they can operate with a supply voltage that significantly exceeds the breakdown voltages of the individual transistors. However, these techniques are only successful over a limited range of input signal swing.
The most common solution for overcoming these limitations is to use an automatic gain control (AGC) circuit in the optical RX module to adjust the gain of the TIA. AGC circuits employ a closed feedback loop having circuitry that measures the amplitude at the output of the TIA and compares it to an output amplitude reference value. If the measured output amplitude is above the reference value, then the gain of the TIA is reduced by the AGC circuit. If the measured output amplitude is below the reference value, then the gain of the TIA is increased by the AGC circuit. This type of AGC circuit architecture can provide a significant increase in the dynamic range of an optical RX module. Such AGC circuits may be implemented only in the TIA or in the entire receiver data path. If the primary motivation is to prevent the TIA from operating in saturation, then the AGC only needs to be implemented in the TIA.
There are challenges and drawbacks associated with typical AGC circuit architectures. One drawback is that it can be difficult for the closed feedback loop to achieve stability. A very low frequency dominant pole is required in the closed feedback loop. The closed feedback loop induces a low frequency cutoff (i.e., a band pass response) that limits the data patterns that can be transmitted. To support longer run lengths of bits, the bandwidth of the feedback loop must be made smaller, which can result in large amounts of silicon real estate being consumed by the AGC circuit.
Another drawback associated with typical AGC circuits is that the mechanism for adjusting the gain is usually a passive resistor in parallel with a metal oxide semiconductor (MOS) active resistor. The MOS active resistor has more parasitic capacitance than the passive resistor, which reduces the upper bandwidth of the receiver and also contributes more noise. Yet another drawback associated with typical AGC circuits is that they have higher power dissipation due to the feedback circuits and the larger bias currents needed to overcome the parasitic capacitance of the variable resistance.
Accordingly, a need exists for a method and an apparatus for controlling the gain of a TIA that eliminate the need for an AGC closed loop feedback circuit.